Can the BBL process be used strictly to shape an already big buttocks?

Without photos or an exam, it is impossible to provide specific feedback to your question. You may need an inferior buttock lift, posterior lower body lift, and liposuction.  Consult with a board certified PS who is an expert in all three procedures.

Hello dear!
Thanks for the question and provided information as well.
Definitely yes!
Fat transfer, along with liposuction allows your plastic surgeon to transform your entire shape. You can improve your abdomen, hips, back and flanks, and of course your buttock, to give you a nice heart shape.
In general, you need to be physically healthy and at a stable weight, have realistic expectations, be a non-smoker and your BMI needs to be 30 or less.
l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)